The tissue expansion technique is one of the most important innovations in skin reconstructive surgery in human and veterinary medicine. This study investigated horses’ physiological and behavioural responses to subcutaneous implantation of a unidirectional self-inflating anisotropic tissue expander. The tissue expanders were subcutaneously implanted on six horses at three different locations: the frontal region of the head, the lateral side of the right shoulder, and the dorsomedial part of the cannon region of the right forelimb. Prior to the tissue expander implantation, each horse was clinically examined, and the observed vital parameters and behaviour were recorded as baseline data. For six days post implantations, the horses were clinically examined, implantation sites monitored, and the horse’s behaviour was recorded both during the day by visual observation and at night with a video camera. The results showed that all horses tolerated the subcutaneous gradual skin expansion by implantation of the tissue expanders, and all surgical sites healed without any complications. The implantation of the tissue expanders does not affect the vital parameters or overall horse behaviour either during the day or at night. In conclusion, the subcutaneous implantation of unidirectional anisotropic tissue expanders in horses resulted in successful skin growth with no physiological and psychological discomfort; hence skin expansion is a good option to be considered when pursuing equine skin reconstructive surgery.

• Agrawal, K., & Agrawal, S. (2012). Tissue regeneration during tissue expansion and choosing an expander. Indian Journal of Plastic Surgery, 45(1), 7-15. https://doi.org/10.4103/0970-0358.96566

• Al-Majhali, S. H., Khairuddin, N. H., Intan-Shameha, A. R., Radzi, Z., Rahman, M. T., Sapalo, J. T., Mayaki, A. M., & Czernuszka, J. T. (2021). Biomechanical effects of unidirectional expansion using anisotropic expanders in horse skin tissue. Journal of Equine Veterinary Science, 99, 103399. https://doi.org/10.1016/j.jevs.2021.103399

• Al-Majhali, S. H., Khairuddin, N. H., Loqman, M. Y., Radzi, Z., Chen, H. C., Lau, S. F., & Rahman, M. T. (2018). Physical measurement of the expansion rate of anisotropic tissue expander in the skin of the horse. Pertanika Journal of Tropical Agricultural Science, 41(4), 1899 -1904.

• Ashley, F., Waterman‐Pearson, A., & Whay, H. (2005). Behavioural assessment of pain in horses and donkeys: Application to clinical practice and future studies. Equine Veterinary Journal, 37(6), 565-575. https://doi.org/10.2746/042516405775314826

• Budras, K. D., Sack, W. O., Rock, S., Horowitz, A., & Berg, R. (2012). Anatomy of the horse (6th ed.). Schlütersche. https://doi.org/10.1201/9783842683686

• Chandawarkar, R. Y., Cervino, A. L., & Pennington, G. A. (2003). Intraoperative acute tissue expansion revisited: A valuable tool for challenging skin defects. Dermatologic Surgery, 29(8), 834-838. https://doi.org/10.1046/j.1524-4725.2003.29218.x

• Chung, E. L. L., Khairuddin, N. H., Azizan, T. R. P. T., & Adamu, L. (2018). Sleeping patterns of horses in selected local horse stables in Malaysia. Journal of Veterinary Behaviour, 26, 1-4. https://doi.org/10.1016/j.jveb.2018.03.014

• De Lorenzi, M., Swan, M., Easter, C., & Chanoit, G. (2018). Outcome of reconstruction of cutaneous limb defects in dogs using hygroscopic “self‐inflating” tissue expanders. Journal of Small Animal Practice, 59(2), 98-105. https://doi.org/10.1111/jsap.12766

• Dyce, K. M., Sack, W. O., & Wensing, C. J. G. (2009). Textbook of veterinary anatomy (4th ed.). Elsevier Health Sciences.

• Evans, J. W. (2000). Horses: A guide to selection, care, and enjoyment (3rd ed.). Henry Holt and Company.

• Fang, L., Zhou, C., & Yang, M. (2013). Expansion in-situ’ concept as a new technique for expanding skin and soft tissue. Experimental and Therapeutic Medicine, 6(5), 1295-1299. https://doi.org/10.3892/etm.2013.1269

• Fuchs, C., Kiefner, C., Reese, S., Erhard, M., & Wohr, A. (2016). Narcolepsy: Do adult horses really suffer from a neurological disorder or rather from a recumbent sleep deprivation/rapid eye movement (REM)‐sleep deficiency? Equine Veterinary Journal, 48(S50), 9. https://doi.org/10.1111/evj.09_12612

• Greening, L., Shenton, V., Wilcockson, K., & Swanson, J. (2013). Investigating duration of nocturnal ingestive and sleep behaviours of horses bedded on straw versus shavings. Journal of Veterinary Behaviour, 8(2), 82-86. https://doi.org/10.1016/j.jveb.2012.05.003

• Keller, W., Aron, D., Rakich, P., Crowe, D., & Marks, M. (1994). Rapid tissue expansion for the development of rotational skin flaps in the distal portion of the hindlimb of dogs: An experimental study. Veterinary Surgery, 23(2), 31-39. https://doi.org/10.1111/j.1532-950X.1994.tb00439.x

• Mason, G., & Mench, J. (1997). Using behaviour to assess animal welfare. In: M. Appleby & B. O. Hughes (Eds.), Animal welfare (pp. 127-141). CAB International.

• Ninomiya, S., Sato, S., Kusunose, R., Mitumasu, T., & Obara, Y. (2007). A note on a behavioural indicator of satisfaction in stabled horses. Applied Animal Behaviour Science, 106(1-3), 184-189. https://doi.org/10.1016/j.applanim.2006.06.011.

• Pedersen, G. R., Sondergaard, E., & Ladewig, J. (2004). The influence of bedding on the time horses spend recumbent. Journal of Equine Veterinary Science, 24(4), 153-158. https://doi.org/10.1016/j.jevs.2004.03.013

• Price, J., Catriona, S., Welsh, E. M., & Waran, N. K. (2003). Preliminary evaluation of a behaviour–based system for assessment of post–operative pain in horses following arthroscopic surgery. Veterinary Anaesthesia and Analgesia, 30(3), 124-137. https://doi.org/10.1046/j.1467-2995.2003.00139.x

• Ransom, J. I., & Cade, B. S. (2009). Quantifying equid behaviour - A research ethogram for free-roaming feral horses. US Geological Survey. https://doi.org/10.3133/tm2A9

• Tepole, A. B., Gosain, A. K., & Kuhl, E. (2012). Stretching skin: The physiological limit and beyond. International Journal of Non-Linear Mechanics, 47(8), 938-949. https://doi.org/10.1016/j.ijnonlinmec.2011.07.006

• Theoret, C., & Schumacher, J. (2017). Equine wound management (3rd ed.). John Wiley & Sons, Inc. https://doi.org/10.1002/9781118999219

• Torcivia, C., & Mcdonnell, S. (2020). Equine discomfort ethogram. Animals, 11(2), 580. https://doi.org/10.3390/ani11020580

• Velnar, T., Bailey, T., & Smrkolj, V. (2009). The wound healing process: An overview of the cellular and molecular mechanisms. Journal of International Medical Research, 37, 1528-1542. https://doi.org/10.1177/147323000903700531

• Werhahn, H., Hessel, E. F., Bachhausen, I., & Van den Weghe, H. F. (2010). Effects of different bedding materials on the behaviour of horses housed in single stalls. Journal of Equine Veterinary Science, 30(8), 425-431. https://doi.org/10.1016/j.jevs.2010.07.005

• Whittaker, C. J., Reynolds, B. D., McCarthy, P. M., Taylor, S. F., Major, D., Caruso, K. A., & Smith, J. (2020). Use of a chronic soft tissue expansion device to facilitate blepharoplasty in a horse with lower‐lid cicatricial ectropion with a 14‐year follow‐up. Veterinary Ophthalmology, 23(5), 899-904. https://doi.org/10.1111/vop.12806

• Yarnell, K., Hall, C., & Billett, E. (2013). An assessment of the aversive nature of an animal management procedure (clipping) using behavioural and physiological measures. Physiology and Behavior, 118, 32-39. https://doi.org/10.1016/j.physbeh.2013.05.013

• Zöllner, A. M., Tepole, A. B., Gosain, A. K., & Kuhl, E. (2012). Growing skin: Tissue expansion in pediatric forehead reconstruction. Biomechanics and Modeling in Mechanobiology, 11(6), 855-867. https://doi.org/10.1007/s10237-011-0357-4

• Agrawal, K., & Agrawal, S. (2012). Tissue regeneration during tissue expansion and choosing an expander. Indian Journal of Plastic Surgery, 45(1), 7-15. https://doi.org/10.4103/0970-0358.96566

• Al-Majhali, S. H., Khairuddin, N. H., Intan-Shameha, A. R., Radzi, Z., Rahman, M. T., Sapalo, J. T., Mayaki, A. M., & Czernuszka, J. T. (2021). Biomechanical effects of unidirectional expansion using anisotropic expanders in horse skin tissue. Journal of Equine Veterinary Science, 99, 103399. https://doi.org/10.1016/j.jevs.2021.103399

• Al-Majhali, S. H., Khairuddin, N. H., Loqman, M. Y., Radzi, Z., Chen, H. C., Lau, S. F., & Rahman, M. T. (2018). Physical measurement of the expansion rate of anisotropic tissue expander in the skin of the horse. Pertanika Journal of Tropical Agricultural Science, 41(4), 1899 -1904.

• Ashley, F., Waterman‐Pearson, A., & Whay, H. (2005). Behavioural assessment of pain in horses and donkeys: Application to clinical practice and future studies. Equine Veterinary Journal, 37(6), 565-575. https://doi.org/10.2746/042516405775314826

• Budras, K. D., Sack, W. O., Rock, S., Horowitz, A., & Berg, R. (2012). Anatomy of the horse (6th ed.). Schlütersche. https://doi.org/10.1201/9783842683686

• Chandawarkar, R. Y., Cervino, A. L., & Pennington, G. A. (2003). Intraoperative acute tissue expansion revisited: A valuable tool for challenging skin defects. Dermatologic Surgery, 29(8), 834-838. https://doi.org/10.1046/j.1524-4725.2003.29218.x

• Chung, E. L. L., Khairuddin, N. H., Azizan, T. R. P. T., & Adamu, L. (2018). Sleeping patterns of horses in selected local horse stables in Malaysia. Journal of Veterinary Behaviour, 26, 1-4. https://doi.org/10.1016/j.jveb.2018.03.014

• De Lorenzi, M., Swan, M., Easter, C., & Chanoit, G. (2018). Outcome of reconstruction of cutaneous limb defects in dogs using hygroscopic “self‐inflating” tissue expanders. Journal of Small Animal Practice, 59(2), 98-105. https://doi.org/10.1111/jsap.12766

• Dyce, K. M., Sack, W. O., & Wensing, C. J. G. (2009). Textbook of veterinary anatomy (4th ed.). Elsevier Health Sciences.

• Evans, J. W. (2000). Horses: A guide to selection, care, and enjoyment (3rd ed.). Henry Holt and Company.

• Fang, L., Zhou, C., & Yang, M. (2013). Expansion in-situ’ concept as a new technique for expanding skin and soft tissue. Experimental and Therapeutic Medicine, 6(5), 1295-1299. https://doi.org/10.3892/etm.2013.1269

• Fuchs, C., Kiefner, C., Reese, S., Erhard, M., & Wohr, A. (2016). Narcolepsy: Do adult horses really suffer from a neurological disorder or rather from a recumbent sleep deprivation/rapid eye movement (REM)‐sleep deficiency? Equine Veterinary Journal, 48(S50), 9. https://doi.org/10.1111/evj.09_12612

• Greening, L., Shenton, V., Wilcockson, K., & Swanson, J. (2013). Investigating duration of nocturnal ingestive and sleep behaviours of horses bedded on straw versus shavings. Journal of Veterinary Behaviour, 8(2), 82-86. https://doi.org/10.1016/j.jveb.2012.05.003

• Keller, W., Aron, D., Rakich, P., Crowe, D., & Marks, M. (1994). Rapid tissue expansion for the development of rotational skin flaps in the distal portion of the hindlimb of dogs: An experimental study. Veterinary Surgery, 23(2), 31-39. https://doi.org/10.1111/j.1532-950X.1994.tb00439.x

• Mason, G., & Mench, J. (1997). Using behaviour to assess animal welfare. In: M. Appleby & B. O. Hughes (Eds.), Animal welfare (pp. 127-141). CAB International.

• Ninomiya, S., Sato, S., Kusunose, R., Mitumasu, T., & Obara, Y. (2007). A note on a behavioural indicator of satisfaction in stabled horses. Applied Animal Behaviour Science, 106(1-3), 184-189. https://doi.org/10.1016/j.applanim.2006.06.011.

• Pedersen, G. R., Sondergaard, E., & Ladewig, J. (2004). The influence of bedding on the time horses spend recumbent. Journal of Equine Veterinary Science, 24(4), 153-158. https://doi.org/10.1016/j.jevs.2004.03.013

• Price, J., Catriona, S., Welsh, E. M., & Waran, N. K. (2003). Preliminary evaluation of a behaviour–based system for assessment of post–operative pain in horses following arthroscopic surgery. Veterinary Anaesthesia and Analgesia, 30(3), 124-137. https://doi.org/10.1046/j.1467-2995.2003.00139.x

• Ransom, J. I., & Cade, B. S. (2009). Quantifying equid behaviour - A research ethogram for free-roaming feral horses. US Geological Survey. https://doi.org/10.3133/tm2A9

• Tepole, A. B., Gosain, A. K., & Kuhl, E. (2012). Stretching skin: The physiological limit and beyond. International Journal of Non-Linear Mechanics, 47(8), 938-949. https://doi.org/10.1016/j.ijnonlinmec.2011.07.006

• Theoret, C., & Schumacher, J. (2017). Equine wound management (3rd ed.). John Wiley & Sons, Inc. https://doi.org/10.1002/9781118999219

• Torcivia, C., & Mcdonnell, S. (2020). Equine discomfort ethogram. Animals, 11(2), 580. https://doi.org/10.3390/ani11020580

• Velnar, T., Bailey, T., & Smrkolj, V. (2009). The wound healing process: An overview of the cellular and molecular mechanisms. Journal of International Medical Research, 37, 1528-1542. https://doi.org/10.1177/147323000903700531

• Werhahn, H., Hessel, E. F., Bachhausen, I., & Van den Weghe, H. F. (2010). Effects of different bedding materials on the behaviour of horses housed in single stalls. Journal of Equine Veterinary Science, 30(8), 425-431. https://doi.org/10.1016/j.jevs.2010.07.005

• Whittaker, C. J., Reynolds, B. D., McCarthy, P. M., Taylor, S. F., Major, D., Caruso, K. A., & Smith, J. (2020). Use of a chronic soft tissue expansion device to facilitate blepharoplasty in a horse with lower‐lid cicatricial ectropion with a 14‐year follow‐up. Veterinary Ophthalmology, 23(5), 899-904. https://doi.org/10.1111/vop.12806

• Yarnell, K., Hall, C., & Billett, E. (2013). An assessment of the aversive nature of an animal management procedure (clipping) using behavioural and physiological measures. Physiology and Behavior, 118, 32-39. https://doi.org/10.1016/j.physbeh.2013.05.013

• Zöllner, A. M., Tepole, A. B., Gosain, A. K., & Kuhl, E. (2012). Growing skin: Tissue expansion in pediatric forehead reconstruction. Biomechanics and Modeling in Mechanobiology, 11(6), 855-867. https://doi.org/10.1007/s10237-011-0357-4